Do you keep your boat in the water at a marina? If so it could be from stray current/electrolysis. Also could be burn marks from your prop if you have an irregularity on one of the leading edges. Any pics?

Here is a pic of my damage. The worst spot (near the leading edge) was probably 1/8" deep. Eric @ OJ Props made short work out of it and it looks as good as new. Fixing the barely noticeable bent blade made sure the damage didn't come back.